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<a name="variadic_macro_data.vmd_modifiers.vmd_modifiers_single"></a><a class="link" href="vmd_modifiers_single.html" title="Modifiers and the single-element sequence">Modifiers
      and the single-element sequence</a>
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<p>
        A single element sequence is what we normally think of when working with
        macro data. It is a single type of macro data passed as an input parameter
        to some macro and processed as such.
      </p>
<p>
        In its basic form without modifiers BOOST_VMD_ELEM serves to just return
        a particular element of a sequence. For a single element sequence BOOST_VMD_ELEM
        with element number 0, just returns the single-element sequence itself. This
        does not offer much functionality for our simple sequence. However with modifiers
        we can do things generically with our single-element sequence which correspond
        to working with a single type of data and extracting information about it.
      </p>
<p>
        With the return type modifier we can get the type of the data along with
        the data. Of course we can also use BOOST_VMD_GET_TYPE to retrieve just the
        type of data.
      </p>
<p>
        With our filter modifier we can retrieve the data only if it is a particular
        type, else retrieve emptiness.
      </p>
<p>
        With the identifier modifier we can retrieve an identifier only if it matches
        one or more other identifiers, else retrieve emptiness.
      </p>
<p>
        With our index modifier we can retrieve both our identifier and its numeric
        index if it matches one or more other identifiers, else retrieve a tuple
        of two empty elements if no match is found.
      </p>
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